"Tat" — That. By uttering this word, seekers of moksha perform all kinds of sacrifice, austerity, and charity without any desire for personal reward. The word "Tat" redirects the entire act toward Brahman.
When a person says "Tat" before an offering, they are declaring: this is not for me. This is for That — the supreme reality beyond name and form. The act stops being personal. It becomes an offering to the infinite.
Seekers of liberation use "Tat" to dissolve the ego's claim over their good deeds. Once an act is surrendered to Brahman, it no longer creates karmic bondage. The doer remains free.